Question

Do enzymes affect the equilibrium constant of a reaction? Why or why not? Use the concepts of thermodynamics versus kinetics to formulate your answer.

Answer 1

The answer is no. Catalysts cannot affect equilibrium constant( K_eq) of a reaction. But they do affect the rate constant(k) of a reaction.

You can think K_eq as the thermodynamic factor and k as the kinetic factor of a reaction.

From kinetic point of view:

The rate constant, k, measures how fast a chemical reaction reaches equilibrium . The reaction must have some sort of energy input before it can proceed; otherwise, the reactants cannot cross the activation energy (E_a) threshold and convert to products. The lower the activation energy for a reaction, the faster the rate. Enzymes speed up reactions by lowering activation energy.

Remember the equation: k = Ae^-Ea/RT

Catalysts decreases the value of Ea . Thus, catalysts increase the rate of reaction. k (rate constant) is affected by catalysts.

From thermodynamic point of view:

Thermodynamics of a reaction describes the free energy between the reactants and the products. Enzymes do not change the equilibrium constant for a reaction. K_eq depends only on the difference in energy level between reactants and products. Enzymes also do not change ?G for a reaction. As shown in the graphs above, enzymes only lower activation energy, but do not change the difference in energy levels between reactants and products.

So catalysts only affect the speed of a reaction (the kinetic factor) . They cannot convert a nonspontaneous reaction into a spontaneous one; catalysts don't drive reactions . The equilibrium of a reaction is determined solely by the difference in energy between the reactant and the product. The thermodynamics of a reaction is not affected by catalysts.